As the third-generation semiconductor material, gallium nitride has features of large band gap width, high electron saturation drift velocity, high breakdown field intensity, good thermal conductivity and so on. In view of electronic devices, gallium nitride material is more suitable for manufacturing semiconductor devices which can withstand high temperature, high frequency, high voltage and have high power compared with silicon and gallium arsenide.
Since strong two-dimensional electron gas exists in an AlGaN/GaN heterojunction structure, High Electron Mobility Transistors (HEMTs) using AlGaN/GaN heterojunction structures are depletion devices and are difficult to be used to implement enhanced devices. However, depletion devices have certain limitations in application in many cases, for example, they cannot be used as power switching devices which are required to be enhanced (normally-off) switching devices. Enhanced gallium nitride switching devices are mainly used for high-frequency devices, power switching devices and digital circuits, thus research on them is important.
In order to implement an enhanced gallium nitride switching device, it is necessary to find some ways to reduce concentration of carriers in a channel below a gate electrode in the case of zero gate voltage. One way is to use an etched structure at the gate electrode to locally decrease a thickness of an aluminum gallium nitride layer below the gate electrode, thereby controlling or reducing concentration of two-dimensional electron gas below the gate electrode. As shown in FIG. 1, a buffer layer 11, a gallium nitride layer 12 and an aluminum gallium nitride layer 13 are located on a substrate 10, a gate electrode 14, a source electrode 15 and a drain electrode 16 are located on the aluminum gallium nitride layer 13. Here, a portion of the aluminum gallium nitride layer below the gate electrode 14 is locally etched, so as to decrease a thickness of a portion of the aluminum gallium nitride layer in a gate region. Another way is to selectively remain p-type (Al)GaN below a gate electrode. The p-type (Al)GaN is used to increase conduction band energy level at an aluminum gallium nitride/gallium nitride heterojunction, so as to form a depletion region and thus implement an enhanced device. As shown in FIG. 2, local p-type nitrides 17 are selectively remained below a gate electrode 14′. Still another way is to utilize fluoride plasma processing technology. Negatively charged ions such as fluorine ions are injected into a barrier layer. Concentration of injected ions can be controlled to deplete two-dimensional electron gas in a conduction channel, and strong negative ions are required to pinch off the channel. As shown in FIG. 3, negatively charged ions 18 are injected into a barrier layer 13 below a gate electrode 14″.
However, these ways have some shortcomings. In the first way, a threshold voltage is generally about 0-1 V which is less than a threshold voltage of 3-5 V for an application. In order to reach a high threshold voltage and a high operating voltage, it is required to form an additional dielectric layer, such as aluminum oxide deposited in an atomic layer, however, how to control an interface state between such a dielectric layer and an aluminum gallium nitride surface is a major problem unresolved. In the second way, it is necessary to selectively etch away all areas except for those below the gate electrode, how to precisely control an etching thickness is very challenging. In addition, defects caused by etching and magnesium atoms remaining in p-type aluminum gallium nitride will result in a severe current collapse effect. Furthermore, due to insufficient hole density (generally a density of holes in p-type gallium nitride is no more than 1E18/cm3), density of two-dimensional electron gas in an AlGaN/GaN heterojunction structure is greatly limited. If there is an excessively high density of electron in the two-dimensional electron gas, an enhanced device cannot be implemented, thus in the AlGaN/GaN heterojunction structure, content of aluminum is generally less than 20%, such as about 15%. In the third way, the fluoride plasma processing will destroy crystal structure, meanwhile, there is a poor processing repeat controllability, which has a large impact on stability and reliability of devices.
Therefore, in view of the above-mentioned technical problems and improvement methods, it is required to provide a new method of manufacturing an enhanced device.